Levels of allowable interferenceLevels of allowable interference

Levels of allowable interference

Whenever any new service is added into an existing service arrangement there will be some increase, even if it is very small, in the interference received by the existing services. In assessing whether the general objective that ‘there should be no noticeable increase in interference to other services’ is satisfied, in terms of practical engineering, the task is to set guidelines that ensure that any increase in interference is constrained to a level that would not adversely impact on existing services. These guidelines have quantified the amounts of interference that would be allowable for the cases of (i) interference to analog television services, and (ii) interference to digital television services. The signal degradation characteristics of analog and digital television systems have been taken into account in determining these limits.

No attempt has been made to quantify levels of interference that would be allowable for other licensed radiocommunication services (including broadcasting services other than analog and digital television). Unlike television‑to‑television interference mechanisms where the characteristics of the device potentially suffering interference can be fairly readily defined, potential interference mechanisms between television and services other than television are much less amenable to detailed specification because the range and characteristics of the devices that could potentially suffer interference is large and diverse. This problem is particularly complex when potential interference mechanisms between services operating in different bands of the radiocommunication spectrum are considered. The current interference management process, which is complaints based and prefaced on the incoming service resolving any interference to existing services, appears to have worked adequately without the need for inclusion of such detailed definitions within the TPGs.

For analog television where the ratio of wanted to unwanted (digital) signals is 41 dB or lower, a degradation of no more than 2 dB in the co‑channel carrier‑to‑noise ratio measured at the receiver input terminals has been permitted. This 2 dB degradation value is based on the interfering signal causing no more than 0.5 grades of degradation on a 5‑grade impairment scale. The basis for this is that the human perception descriptors applicable to impairments to planned television reception (grade 5 – imperceptible; grade 4 – perceptible, but not annoying; grade 3 – slightly annoying) are fairly closely spaced and it seems reasonable to assume that defining a 0.5 grade change as an allowance for interference will not be perceived as having a noticeable impact on the level of interference to the signal. The relationship between subjective impairment gradings and carrier‑to‑noise (or unweighted video‑signal‑to‑noise) ratio is given in texts such as ITU‑R Recommendation BT.654 or Hutson, G.H. – Colour Television: system principles, engineering practice, and applied technology, 2nd Ed, McGraw Hill 1990.